1. Field of the Invention
The present invention relates to a structure of magnetic disk apparatus. More specifically, the present invention relates to a structure of a thin type clamp for fixing magnetic disks to spindle hubs.
2. Description of the Background Art
FIG. 15 is a perspective view showing an appearance of a magnetic disk apparatus. To spindle hub 2 rotated by spindle motor 5, one or a plurality of disk 1 are laminated and fixed at even intervals and on both surfaces or one surface of disk 1, head 21 is rotatably arranged. This head 21 is fixed to the head end section of actuator 22, and to the other end of actuator 22, voice coil motor 24 is installed. As spindle motor 2 rotates, head 21 slightly floats up from disk 1 by air stream generated between the surface of disk 1 and head 21.
On the other hand, head 21 turns around actuator shaft 23 by a driving force of voice coil motor 24, travels on disk 1 in an radial direction, and reads and writes data. Because the float-up amount of head 21 is about few score nanometers, and entry of dust and grime between head 21 and disk 1 gives damage to head 21 and disk 1 and causes troubles, magnetic disk apparatus is assembled in a clean room, and after the assembly, it is hermetically sealed with cover 25. In order to correctly read and write the data, head 21 must accurately follow a data track formed helically on disk 1, and there are severe restrictions in the radial direction and an axial direction with respect to rotating deviation of disk 1. Main rotating deviation of disk 1 includes:
(1) rotating synchronous/asynchronous deviation arising from bearing of spindle motor 5 and its mounting;
(2) rotating synchronous deviation arising from processing errors of spindle hub 2;
(3) rotating synchronous deviation of disk 1 arising from clamping force when clamp 3 is tightened;
(4) rotating synchronous deviation caused by deviating the data track center and rotating center axis;
(5) rotating asynchronous deviation of spindle motor 5 and disk 1 arising from mechanical resonance; and others.
Because in recent years, the density of magnetic disk increases and a large volume of information has become possible to be recorded on a disk of low recording area, development has been positively taking place for adopting magnetic disk apparatus as recording medium of, for example, mobile equipment. The conditions required for recording medium of mobile equipment include not only small and compact size but also low power consumption and shock resistance from the viewpoint of their characteristics of use.
FIG. 16 shows a cross-sectional view before clamped showing the first clamp structure in a conventional magnetic disk apparatus. Clamp 3 grasps disk 1 between disk depressing section 3g and disk receiving surface 2e by axial force of screw 4. Disk 1 is integrally fixed to spindle hub 2 by frictional force of relevant contact sections. In the event that large shock that exceeds this frictional force is applied, the position of disk 1 fixed to clamp 3 and spindle hub 2 may be greatly deviated. This is called as disk shift and one of the causes to give raise to the rotating deviation of disk 1. When disk shift occurs, large eccentricity is generated in the data track originally provided coaxially with the rotating center axis and it becomes difficult for head 21 to accurately follow the data track.
When a hard disk is used for a recording medium of mobile equipment, shockproof to guarantee normal operation even after shocks exceeding 1500 G are applied at the time of non-operation is frequently required. The following configuration is known for improving the shock resistance. That is, because the frictional force which fixes disk 1 must be increased to prevent disk shift arising from strong shock, methods of increasing the frictional coefficient or of increasing axial force are assumed. Conventionally, a method of increasing axial force has been adopted. This is because the surface roughness must be refined for spindle hub 2 and clamp 3 as their processing accuracy would be required, or burr and adhesion of contaminant, etc. are feared. Furthermore, because the float-up amount of head 21 with respect to disk 1 is few score nanometers and technologies tend to lower the float-up amount, the finer, the more desirable is the surface roughness. Consequently, it was unable to coarsen surface roughness (increase the frictional coefficient) of spindle hub 2, and clamp 3, or disk 1 bottom surface. From a technological point of view, a method of coarsening the surface roughness in the clamping area only excluding the data area is possible, but this would inevitably result in cost increase. Consequently, as a conclusion, a method of increasing axial force has been adopted.
However, as shown in Paragraph (3) above, increasing axial force to improve shock resistance increases rotating synchronous deviation of disk 1 caused by the clamping force when clamp 3 is tightened. This means that conditions for improving shock resistance and conditions for solving rotating synchronous deviation of disk 1 arising from clamping force would conflict each other. Consequently, it is essential to reconcile improvement of shock resistance and elimination of rotating synchronous deviation particularly in developing small-size magnetic disk apparatus.
In order to suppress rotating synchronous deviation of disk 1 arising from increased clamping force to the minimum, it is necessary to hold the center axis of disk depressed section coaxial with the disk receiving surface of spindle hub and disk center axis. Referring now to two kinds of clamp structure in conventional magnetic disk apparatus, these relations are explained.
FIGS. 16 and 17 show the first clamp structure in conventional magnetic disk apparatus. Referring to FIG. 16, disk 1 is inserted in disk inserting boss 2a of spindle hub 2. One of the bottom surfaces of disk 1 is received by disk receiving surface 2e. Clamp 3 is mounted on the other bottom surface of disk 1 coaxially with disk 1, and tightened between screw 4 and female thread 2c provided at boss 2a of spindle hub 2. Since the diameter of thread head section 4b of screw 4 is greater than the diameter of central hole 3a of clamp 3, axial force is generated when screw 4 is tightened to female thread 2c of spindle hub 2. The axial force is transmitted to bottom surface 3f in the vicinity of clamp central hole 3a from thread head section bearing surface 4c, presses the other bottom surface of disk 1 at disk depressing section 3g, and allows clamp 3 to coaxially and integrally fix disk 1 to spindle hub 2.
In order to coaxially mount disk 1 to spindle hub 2, the central hole of disk 1 and peripheral cylindrical section of disk insertion boss 2d of spindle hub 2 slightly smaller than the central hole diameter of disk 1 are positioned and fitted to each other. In addition, in order to coaxially mount clamp 3 and spindle hub 2, clamp positioning boss 2a of spindle hub 2 and clamp central hole 3a with a diameter slightly greater than the diameter of clamp positioning boss 2a are positioned and fitted to each other. Positioning is achieved by arranging clamp positioning boss 2a and clamp 3 in such a manner to have a catching portion of length a (see enlarged view of FIG. 16). The catching portion is provided to prevent clamp 3 from deviating in the direction perpendicular to the center axis. Clamp 3 bends by thickness b with length the catching portion excluded, and is tightened by screw 4.
Note that, boss 2a of spindle hub 2, disk insertion boss 2d, and disk receiving surface 2e hold coaxial with the rotating center axis, respectively. On the other hand, disk depressing section 3g of clamp 3 and clamp central hole 3a also hold coaxial with the rotating center axis. That is, disk depressing section 3g holds coaxial with disk receiving surface 2e of spindle hub 3 and disk 1. FIG. 17 shows a cross-sectional view after clamped in the conventional magnetic disk apparatus. FIG. 17 shows how the above-mentioned component elements hold coaxial with the rotating center axis.
Referring now to FIG. 18, description will be made on the necessity why disk depressing section 3g is held coaxial with disk receiving surface 2e of spindle hub 3 and disk 1. FIG. 18 is a cross-sectional view showing magnetic disk apparatus with deflection of disk generated due to the clamp structure with coaxiality not maintained. From the figure, it is understood that spindle hub center axis 2h and clamp center axis 3q are deviated. This is because unbalance of load working point to the load back face is generated due to non-uniform load distribution to disk depressing section 3g by the axial force from screw head bearing surface 4c transmitted eccentrically to clamp central hole 3a and disk depressing section 3g being eccentric to disk 1 and disk receiving surface 2e, and disk 1 causes large camber in the axial force direction on the eccentric direction side of clamp 3, while on the opposite side of the eccentric direction large camber is generated in the direction opposite to the axial force. If spindle motor 5 rotates under this condition, rotating synchronous deviation results. Consequently, it is necessary to hold disk depressing section 3g concentric with disk receiving surface 2e of spindle hub 3 and disk 1.
FIGS. 19A and 19B show the second clamp structure in conventional magnetic disk apparatus. FIG. 19A is a cross-sectional view before clamped showing the second clamp structure in conventional magnetic disk apparatus. FIG. 19B is a cross-sectional view after clamped, showing the second clamp structure in conventional magnetic disk apparatus. Since the configuration of disk 1, spindle hub 2, clamp 3, screw 4, and female screw 2c are the same as those in the first clamp structure (FIG. 16, 17), the description will be omitted.
Now, in order to make disk 1 concentric with spindle hub 2, the central hole of disk 1 and peripheral cylindrical section of disk insertion boss 2d of spindle hub 2 slightly smaller than central hole diameter of disk 1 must be positioned and fitted in. Consequently, in the second clamp structure, two or more non-through holes 2f are provided at equal angle intervals on spindle hub 2 and at the same time on a coaxial pitch circle with the rotating center axis of spindle hub 2. Non-through holes 2f of spindle hub 2 and hole 3o of clamp 3 are arranged in such a manner that the relevant centers are overlapped, jig pin 20 is inserted into the overlapped portion, and non-through hole 2f, hole 3o, and jig pin 20 are fitted in. That is, non-through holes 2f, hole 3o, and jig pin 20 are means for positioning clamp 3 with respect to spindle hub 2.
Note that, non-through holes 2f formed at 2 or more places at equal angle intervals of spindle hub 2, disk inserting boss 2d, and disk receiving surface 2e are kept coaxial with the rotating center axis, respectively. On the other hand, disk depressing section 3g of clamp 3 and clamp central hole 3a are kept coaxial. That is, disk depressing section 3g is kept coaxial with disk receiving surface 2e of spindle hub 3 and disk 1.
Next, discussion will be made on reasons why conventional second clamp structure (FIG. 19) is chosen. FIG. 20 shows a cross-sectional view of screw with screw driving hole and screw head reinforced. In the event that disk apparatus is made still thinner, thickness of screw head section 4b must be made still thinner, and screw head reinforcement 4d must be provided for forming screw driving hole 4e. In the case of the first clamp structure (FIG. 16, FIG. 17), clamp positioning boss 2a has been provided to spindle hub 2. Because as screw head section reinforcement 4d increases, the diameter of boss 2a must be increased to prevent interference between inner circumferential hole provided and screw head reinforcement 4, diameters of clamp central hole 3a and screw head 4d must also be increased. However, the diameter of screw head section 4d becomes more and more difficult to fabricate because of restrictions of thread head section forming.
When the second clamp structure (FIG. 19) and first clamp structure (FIG. 16, FIG. 17) are compared, the diameter of screw head section 4d can be reduced in the second clamp structure (FIG. 19) as much as clamp positioning boss 2a which is no longer required. That is, in the event that a still thinner type is pursued, the second clamp structure (FIG. 19) is advantageous. The clamp thickness can be reduced to the degree in which characteristics as a spring will not be lost, that is, to the degree in that no plastic deformation occurs when clamping force is applied.
As described above, the clamp structure in magnetic disk apparatus must satisfy conflicting requirements of reducing thickness, securing freedom of disk shift against guaranteed vibration and shock values, and reducing camber, undulation, etc. of disk 1 generated by eccentricity between disk depressing section and disk receiving surface of spindle hub and disk.
Now, description will be made on problems which the first clamp structure (FIG. 16, FIG. 17) and the second clamp structure (FIG. 19) have in conventional magnetic disk apparatus.
First of all, problems of the first clamp structure (FIG. 16, FIG. 17) are described as follows. In general, clamp disk 9 is manufactured of aluminum or stainless steel. For a processing method, cutting using press, lathes, etc. is adopted, and in any case, it is possible to produce a large quantity in a short time, and it is a processing method with primary emphasis placed on cost. For clamp 3 in the first clamp structure shown in FIG. 16 and FIG. 17, the method of fabricating using press would be best-suited for mass production. From the clamp structure, the following relation must be found:(Thickness t of clamp 3)=x+y  (Eq. I),
where x is the catching portion of clamp central hole 3a and clamp positioning boss 2a and y is a deflection amount of clamp 3. Furthermore, for clamping force N of clamp 3, the following relation must be found:Clamping force N=K*y  (Eq. II),
where K is an axial spring constant of clamp 3.
As described above, since clamp 3 as one of components, is required to reduce its size and thickness, in particular, for small-size magnetic disk apparatus, t must be reduced from Eq. I. The catching portion x greater than a specified amount is definitely needed for positioning clamp 3. Consequently, deflection amount y must be reduced. However, if deflection amount y is reduced, axial force is reduced.
However, large axial force, that is, large clamping force is also required for a small-size magnetic disk apparatus. That is, according to Eq. II, N must be kept large. K is a constant that is defined by material, shape, and thickness. It is difficult to increase thickness from the viewpoint of a requirement of reducing thickness. Consequently, the deflection rate y must be increased.
The foregoing description indicates that the conditions required for deflection amount y contradict each other in Eq. I and Eq. II. Under the current requirement for clamp, it is difficult to achieve reduced size and thickness of clamp 3 and increase clamping force simultaneously.
Next description will be made on problems in the second clamp structure (FIG. 19) as follows. The eccentricity between disk depressing section 3g of clamp 3 and disk receiving surface 2e is qualitatively compared with the first clamp structure (FIG. 16). The smaller the eccentricity, the smaller are camber and undulation of disk 1.
In the first clamp structure (FIG. 16, FIG. 17), eccentricity between disk depressing section 3g (FIG. 16) of clamp 3 and disk receiving surface 2e (FIG. 16) is caused by the following size and accumulation of tolerances. That is,
(1) A clearance between clamp positioning boss 2a (FIG. 16) and clamp central hole 3a (FIG. 16),
(2) Eccentricity between clamp positioning boss 2a and disk receiving surface 2e, and
(3) Eccentricity between clamp central hole 3a and disk depressing section 3g. 
On the other hand, in the second clamp structure (FIG. 19), eccentricity of disk depressing section 3g of clamp 3 (FIG. 19) and disk receiving surface 2e (FIG. 19) is generated at least following size and accumulation of tolerances.
(1) Eccentricity of pitch circle center and disk depressing section 3g of clamp hole 3o (FIG. 19).
(2) Variations of pitch circle diameter of clamp hole 3o. 
(3) Variations of diameter of clamp hole 3o. 
(4) Eccentricity of pitch circle center of non-through hole 2f (FIG. 19) and disk receiving surface 2e. 
(5) Variations of pitch circle diameter of non-through hole 2f. 
(6) Variations of a diameter of non-through hole 2f. 
As described above, in the second clamp structure (FIG. 19), eccentricity of disk depressing section 3g of clamp 3 and disc receiving surface 2e increases and camber and undulation of disk 1 increase. To prevent this, in fabricating parts, severe processing accuracy is required, and this results in cost increase.
Furthermore, in inserting jig pin 20 (FIG. 19), in order to improve the positioning accuracy of clamp 3, play generated between parts must be suppressed to the minimum, and degraded operability and trouble in assembly will result. Specifically, displacement of clamp 3 (FIG. 19) generated when screw 4 (FIG. 19) is tightened to spindle hub 2 (FIG. 19) is generated not only in the axial direction but also in the radial direction. Consequently, there are cases in which jig pin 20 is grasped between side surface of hole 3o of clamp 3 and side surface of non-through hole 2f of spindle hub 2, and is difficult to be removed. As the axial displacement of clamp 3 increases, radial displacement increases. That is, the greater is the axial force, the more degraded is the operability in assembly and the more increased are troubles.